The present invention relates to a method and apparatus for inspecting for defects on the surface of a substrate for a magnetic disk and a semiconductor wafer. In particular, the invention relates to a surface defect inspection method and apparatus suitable for optically inspecting the surface of a substrate to detect dents and scratches on the surface and foreign matters attached to the surface.
An apparatus for optically inspecting for microscopic defects on the surface of a substrate for a magnetic disk and a semiconductor wafer is required to carry out higher speed and more sensitive inspection. To achieve more sensitive inspection, a method is adopted which includes increasing the intensity of illumination light by using laser as a light source and detecting light reflected from the substrate and scattering with a high sensitivity sensor, as described in, for example, Japanese Published Patent Application No. Hei 3-186739 and Japanese Published Patent Application No. Hei 5-21561. However, when the power or intensity of the illumination light is too strong, it will damage the substrate surface. Hence, reflected and scattering light from a defect on the substrate surface illuminated by the illumination light with limited irradiation intensity has to be detected with maximum detection sensitivity. As a configuration for detecting this reflected and scattering light with maximum detection sensitivity, the configuration adopting a photomultiplier unit is disclosed in Japanese Published Patent Application No. Hei 5-21561.
Scattering light from a microscopic defect has a characteristic in which, as the size of the defect becomes smaller, the amount of the scattering light from the defect per unit area decreases and this scattering light is more likely to diffuse across the whole space above the defect. Therefore, it is necessary to efficiently collect and detect the scattering light from the defect in a wider region in order to detect a microscopic defect sensitively using the illumination light with the same amount of light. To collect the scattering light in a wider region (solid angle), this can be accomplished by using a larger objective lens having a larger numeral aperture (NA) for collecting the scattering light from the substrate. However, employing a larger objective lens having a larger numeral aperture is limited in practice, in order that the objective lens should be prevented from interfering with other parts when installed in the inspection apparatus.
If a larger objective lens having a larger numeral aperture is employed, a converging lens through which the scattering light collected by the objective lens converges on the detector plane of a detector has to be larger accordingly. A set of these larger lenses requires larger lens barrels for supporting them, thus enlarging a detection optics system and making it difficult to make the apparatus smaller and lighter.